CAMP accumulation agonists

The multiplicity of G proteins coupled to opiate receptors may explain how different opiates can bind to the same receptor yet induce different cellular responses. For example, morphine binds to the cloned rat fi receptor expressed in HEK 293, CHO and COS-7 cells and inhibits cAMP accumulation [80-82]. Morphine can be continuously applied to the cells for up to 16 h, and the potency and magnitude of morphine inhibition of adenylyl cyclase does not diminish [80, 81]. In contrast, the opiate sufentanil can bind to the same cloned fi receptor in HEK 293 cells to inhibit cAMP accumulation. However, sufentanil s actions rapidly desensitize [83]. Since both compounds bind to the same receptor, and the fi receptor is the only receptor these drugs can interact with in these cells, the ability of these two full agonists to differentially regulate the fi receptor must be due to their abilities to affect separate adaptive processes in these cells. 

Mutagenesis studies have shown that morphine and sufentanil bind differently to the jj, receptor [83, 85]. Mutation of an aspartic acid at residue 114 of the // receptor to an asparagine resulted in a mutant that did not bind morphine and morphine was ineffective in inhibiting adenylyl cyclase via that receptor. In contrast, sufentanil bound to the mutant and wild-type receptors equally well and it effectively inhibited cAMP accumulation via the mutant receptor. These findings demonstrate that morphine and sufentanil have different requirements for binding to the // receptor. By binding differentially, these two agonists may induce the ft receptor to interact with different G proteins to induce distinct cellular effects. 

While chronic morphine treatment uncouples the // receptor from K+ channels, it did not affect the coupling of ft receptors to adenylyl cyclase. Pretreatment of the cloned ft receptor expressed in HEK 293, AtT-20, CHO and COS cells with morphine or DAMGO for up to 16h did not alter the subsequent ability of fi agonists to inhibit cAMP accumulation [25, 65, 80-82]. These findings suggest that morphine treatment induces a selective desensitization of the coupling of the fi receptor to K+ channels. 

The pupils become dilated and there are associated signs of hyperactivity of the sympathetic nervous system, such as hypertension and pilomotor stimulation. The mechanism(s) underlying tolerance and dependence are poorly understood. While acute activation of Gi/o-coupled receptors leads to inhibition of adenylyl cyclase, chronic activation of such receptors produces an increase in cAMP accumulation, particularly evident upon withdrawal of the inhibitory agonist. This phenomenon, referred to as adenylyl cyclase superactivation, is believed to play an important role in opioid addiction. 

G protein-mediated responses to drugs and hormonal agonists often attenuate with time (Figure 2-12, top). After reaching an initial high level, the response (eg, cellular cAMP accumulation, Na+ influx, contractility, etc) diminishes over seconds or minutes, even in the continued presence of the agonist. This "desensitization" is often rapidly reversible a second exposure to agonist, if provided a few minutes after termination of the first exposure, results in a response similar to the initial response. 

The dopamine-stimulated formation of cAMP may initiate the dopamine-induced release of IR-PTH. A linear relationship exists between the dopamine-induced release of IR-PTH and the logarithm of the dopamine-induced accumulation of cAMP (17). Similarly, other agents increasing cAMP accumulation and IR-PTH release (e.g. beta-adrenergic agonists, secretin and phosphodiesterase inhibitors, also display such a log-linear relationship. Additional support for the possibility that intracellular cAMP might initiate PTH secretion comes from the observations that cholera toxin (JJ.), phosphodiesterase inhibitors (17) and dibutyryl cAMP (18), agents known to increase intracellular cAMP or mimic the biochemical effects of cAMP, increase the release of IR-PTH. 

Figure 9. Specificity of fi-adrenergic and dopaminergic stimulation of cAMP accumulation. Dispersed parathyroid cells were incubated with the indicated fi-adrenergic or dopaminergic agonists either alone (open bars), with 1 pM (—) propranolol (solid bars), or with 10 pM- a-flupenthixol (stippled bars).

ADTN and other dopamine agonists mimicked this effect which was antagonized by a- and B-flupenthixol, the a-isomer being 100 times more potent. In a similar way, dopamine caused a rapid 20-30-fold increase in cellular cAMP in dispersed bovine parathyroid cells. The potency of a series of dopaminergic agonists and antagonists on adenylate cyclase activity paralleled the effects of these ligands on CAMP accumulation and parathormone secretion (16). It was concluded that bovine parathyroid cells possess dopamine sites which are involved in the control of parathormone secretion. 

Secondly ergot derivatives which reveal a clearcut agonistic activity on prolactin secretion and as antiparkinson agents (20) were inactive on the cyclase. Surprisingly, lisuride and lergotrile were found to be weak antagonists of dopamine stimulated cAMP accumulation, but they could also antagonize the cAMP production stimulated by isoproterenol as... 

Although earlier studies failed to find consistent effects of DA on cAMP levels or adenylate cyclase activity in anterior pituitary cells [13], intact pituitary gland [14,15] or homogenates [13,16], a functional connection between the two is now supported by many experimental approaches. DA and DA agonists inhibit cAMP levels in cultured rat pituitary cells at concentrations in the nanomolar range, comparable to those which inhibit PRL release [17-21], DA also inhibits cAMP accumulation stimulated by VIP or TRH [20]. Inhibition is also seen in human prolactinoma cells [22]. 

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